Embargoed until 5:00 p.m. EDT
NSF PR 02-58 - July 8, 2002
Deformed Frogs Form When Parasites and Pesticides
Combine
Deformities in Pennsylvania wood frogs are linked to
a parasite infection combined with a weakened immune
system caused by pesticide exposure, according to
a study to be published in the 9 July issue of the
Proceedings of the National Academy of Sciences.
The research, supported by a grant from the Ecology
of Infectious Diseases Program, and jointly administered
by the National Science Foundation (NSF) and the National
Institutes of Health (NIH), includes the first experimental
studies of amphibian deformities conducted in ponds
where the animals live. The discoveries, which show
the effect of environmental stress on disease outbreaks,
may help to explain how disease affects the distribution,
growth, development, and survival of frogs.
"Frog, toad, and salamander populations have been declining
on a worldwide basis for the past several decades,"
says Sam Scheiner, program director in NSF's division
of environmental biology. "These declines may be a
warning of an overall decline in the health of the
environment. By understanding the link between pesticides
and parasite infection, we can better manage the environment
and improve human health."
It is not uncommon now for 20 to 30 percent of the
frogs at many locations to have limb deformities,
says Joseph Kiesecker, a biologist at Penn State and
the leader of the research team. Since the early 1990s,
when school children and amateur naturalists first
began finding frogs with deformed legs in U. S. wetlands,
scientists have been trying to determine the reason
for the problem's escalation. These deformities in
frogs resemble the deformities in human caused decades
ago by the drug thalidomide. "Both the general public
and scientists suspect that whatever is causing these
problems in frogs may also cause harm to humans,"
Kiesecker says.
The Kiesecker team designed experiments to test hypotheses
regarding the relationship between pesticides, trematode
parasites, and limb deformities in frogs. The first
hypothesis was that limb deformities occur in frogs
infected with the trematode parasite. Trematode parasites
inhabit a series of host species, including pond snails,
during their life cycle. When they leave the snail,
in the form of trematode larvae called cercariae,
they swim around in the pond in search of a tadpole,
which is the next host they need to invade in order
to survive. The researchers placed groups of their
tadpoles in the six ponds within two kinds of enclosures
located side-by-side--one with a fine screen that
prevented the trematode larvae from entering the enclosure,
and the other with a larger-mesh screen that allowed
the trematode larvae to infect the tadpoles.
The only tadpoles that developed limb deformities in
the first experiment were those exposed to the trematode
larvae, while protected tadpoles were not deformed.
"We learned from the first field experiment that tadpoles
have to be exposed to trematode infection for limb
deformities to develop," Kiesecker explains.
The second hypothesis the team tested is that limb
deformities in trematode-infected tadpoles are affected
by pesticides. When they analyzed the rates of limb
deformities among their research animals, they found
much higher rates of deformities in trematode-infected
tadpoles at the three ponds that receive agricultural
runoff and contain pesticides than in the ponds that
do not.
The team then moved into the lab to test their third
hypothesis, which is that pesticide exposure--not
some other factor--influenced the increased rates
of deformities developed by the trematode-infected
tadpoles in the field study. These laboratory experiments
involved three groups of tadpoles that the researchers
exposed to three different pesticides, plus one group
that they did not expose to pesticides. The pesticides
were Atrazine--the most commonly used pesticide in
North America, Malathion--a common household pesticide
that also is used to control insect pests in agricultural
fields, and Esfenvalerate--a synthetic pyrethroid
pesticide. "Synthetic pyrethroids have become increasingly
popular during the last couple of years because they
are not very toxic to birds and mammals; however,
they are highly toxic to many other kinds of organisms,"
Kiesecker says.
In summary, Kiesecker says, "The field experiments
showed that only the tadpoles that were infected with
trematodes developed limb deformities and that these
deformities occurred with more frequency in the groups
of tadpoles that also were exposed to pesticides.
The kicker is that the concentrations that caused
deformities were incredibly low for Esfenvalerate
and Atrazine--low enough for humans to drink, based
on Environmental Protection Agency standards."
PHOTOS: High-resolution images for publication are
available to reporters from a link at http://www.science.psu.edu/alert/Kiesecker7-2002.htm
|